The United States and Canada generate about 100,000,000 pounds of spent base-metal catalyst per year, about half of which is spent hydroprocessing catalysts. The present invention relates to a process for regenerating spent, heavy hydrocarbon, hydroprocessing catalysts. More specifically, the present invention relates to a process for regenerating spent, heavy hydrocarbon, hydroprocessing catalysts that have been deactivated with coke and metal deactivants such as nickel and vanadium.
With respect to the present invention, the term hydroprocessing is used to refer to a process for hydrodemetallation, hydrodesulfurization, hydrodenitrogenation, and hydroconversion wherein the term hydroconversion encompasses the hydrocracking and hydrotreating of hydrocarbon streams containing asphaltenes and contaminant metals. Hydroprocessing catalysts used to treat heavy hydrocarbon streams, such as resids, are deactivated as a result of metals and coke deposition. These deposition materials modify the rate of reaction as well as accelerate the rate of catalyst deactivation. The various metal deposits tend to occlude catalyst pores and poison the hydroprocessing catalyst, while coke deposits similarly reduce the pore size and surface area of the hydroprocessing catalyst.
Typically, hydroprocessing catalysts possess substantial macropore volume in order to effect metals removal from heavy hydrocarbon feed streams. Heavy hydrocarbon hydroprocessing catalysts possess the capacity to adsorb contaminant metals, such as nickel and vanadium, in an amount ranging up to about 100 weight percent of the fresh catalyst weight. However, due to the rapid coke deposition rate, the catalyst is deactivated prior to achieving its full metals adsorption capacity. Such catalysts are taken out of service when they contain as little as 10 weight percent nickel plus vanadium.
Thus, the prior art is replete with processes suitable for handling these spent catalysts. These processes can be separated into four areas: (1) disposal, (2) total catalyst reclamation, (3) catalyst rejuvenation, and (4) catalyst regeneration.
Disposal of spent catalyst requires compliance with stringent environmental standards that can substantially increase the cost of handling spent catalyst. One acceptable method of disposal of spent catalyst is encapsulation, wherein the catalyst is surrounded, either as a pellet or in bulk, with an impervious layer of sealant. Bitumen, paraffin wax, and polyethylene have been used as thermoplastic encapsulating agents in which the dry waste is mixed with the sealant at high temperature and cooled. Similar methods have been developed for low temperature encapsulation, where a polybutadiene binder can be mixed with the catalyst followed by application of a thin polyethylene jacket around the mass. Although encapsulation provides a relatively effective means of disposing of spent catalyst, the possibility of fire and weathering can lead to long term instability of the encapsulants. In addition, care must be taken to avoid shear stresses that can break open the encapsulated particles.
Total catalyst reclamation involves removing both the contaminant and catalytic metals from the spent catalyst for recovery as precious metals. The use of catalyst as a source of precious metals is well known in the industry. Most metal recovery methods involve roasting the spent catalyst in the absence or presence of additives, followed by leaching of the catalyst. A suitable method of recovering precious metals from spent catalyst is presented in G. Parkinson, "Recyclers Try New Ways to Process Spent Catalyst," Chemical Engineering, Feb. 16, 1987, pp. 25-31. This reference discloses a Gulf Chemical and Metallurgical Corporation metal recovery method wherein molybdenum and vanadium are converted to their sodium salts by adding sodium carbonate during a multiple-hearth roast at 650.degree. to 900.degree. C. The spent catalyst is roasted to remove carbon and sulfur and the calcined material quenched in water to dissolve the salts. A leach liquor is then separated from the insolubles by countercurrent decantation. Ammonium chloride is subsequently added to precipitate ammonium vanadate, which is calcined and fused to produce vanadium pentoxide. The remaining solution is then heated to 80.degree. to 85.degree. C. and acidified to precipitate molybdic acid, which is calcined to molybdic oxide. Other examples of metal recovery techniques are disclosed in Trimm, D. L., "Deactivation, Regeneration, and Disposal of Hydroprocessing Catalysts," Catalyst in Petroleum Refining 1989, Elsevier Science Publishers, B.V. Amsterdam (1990).
Catalyst rejuvenation involves selective removal of contaminant metals from the spent catalyst followed by or preceded by oxidative decoking of the coke deposited on the catalyst. The objective in catalyst rejuvenation is to remove the contaminant metals while retaining the catalytic metals on the catalyst for reuse. One way of achieving selective removal of contaminant metals, and in particular vanadium, is by attrition. It is well known that vanadium and iron are deposited on the exterior of some catalysts. As a result, subjecting the outermost layers of the catalyst to abrasion leads to a powder rich in coke, vanadium and iron, while the residual pellet contains catalytic material and nickel (which is deposited throughout the pellet). Although some improvement in activity is observed, the overall benefit is not high because abrasion of the exterior deposits does not generally remove the vanadium that is deposited in the pore mouths of the catalyst. Deeper abrasion to remove pore-mouth deposits can weaken the catalyst. Another approach for selectively removing contaminant metals from the spent catalyst is selective leaching which takes advantage of the fact that metals on the spent catalyst are usually present as sulfides. Selective leaching involves treating the spent hydroprocessing catalyst with a chemical that reacts with only one of these sulfides, e.g. the use of oxalic acid to remove vanadium. An extensive discussion of the use of leaching to selectively remove metals from spent hydroprocessing catalyst can be found in M. Marafi, A. Stanislaus, C. J. Mumford, and M. Fahim, "Regeneration of Spent Hydroprocessing Catalyst: Metal Removal," Applied Catalysis, 47(1989) pp. 85-96.
The problem with such selective removal processes is that invariably, some of the catalytic metals are also removed, thereby requiring that at least a portion of the catalytic metals be reincorporated onto the catalyst prior to reuse. The problems associated with selective removal of contaminant metals have lead some operators in the industry to consider catalytic regeneration of the spent catalyst. In other words, decoking the spent, metal-containing hydroprocessing catalyst using oxidative combustion, retaining the contaminant and catalytic metals until the metals build-up is so severe that the catalyst needs to be replaced. Regeneration takes into account that deactivation due to coking generally occurs much more rapidly than deactivation due to metal deposition. In addition, the catalyst's demetallization capacity is more fully utilized, thereby increasing its eventual reclamation value. However, efforts to commercialize the regeneration of resid hydroprocessing catalysts have been largely unsuccessful.
A problem with regeneration by decoking in the presence of contaminant metals, and in particular vanadium, is that the catalyst becomes soft, i.e., the catalyst's resistance to attrition is significantly reduced. This problem is particularly severe when the catalyst is employed in an ebullated bed reaction process where the solid catalyst particles are kept in random motion by the upward flow of liquid and gas. Similarly, equipment commonly used with ebullated bed reaction systems such as the ebullated bed pumps further aggravate catalyst attrition. These physical process factors attendant to resid hydroprocesses combined with the fact that large pore resid hydroprocessing catalysts can be particularly vulnerable to attrition per se, make the attrition resistance of the catalyst critically important.
Moreover, it has now been found that when a spent, contaminant metal-containing, hydroprocessing catalyst is subjected to oxidative combustion, one of the species oxidized is vanadium sulfide, V.sub.3 S.sub.4, the predominant vanadium phase deposited under typical hydroprocessing conditions. The vanadium sulfide (V.sub.3 S.sub.4) is then oxidized to vanadium pentoxide (V.sub.2 O.sub.5), a highly oxidative catalyst, during the combustion step which is highly detrimental to resid hydroprocessing catalyst attrition properties due to at least two separate and distinct reaction mechanisms.
The first detrimental mechanism proceeds from the reaction of the water formed during the oxidation of carbonaceous deposits with the vanadium pentoxide formed by the oxidation of vanadium sulfide to vanadium pentoxide. This reaction forms vanadic acid, VO(OH).sub.3, a volatile and highly reactive species that reacts with metals present in the catalyst such as iron, nickel, aluminum or molybdenum to form mixed metal vanadates. These mixed metal vanadates generally cause loss of both catalyst surface area and attrition resistance.
The second detrimental mechanism occurs since vanadium pentoxide also catalyzes the conversion of sulfur dioxide formed during the combustion reaction to reactive sulfur trioxide, an acidic gas. The sulfur trioxide generally reacts with alumina supports, resulting in loss of both surface area and attrition resistance as taught in U.S. Pat. Nos. 4,089,806 and 4,994,423 respectively.
It is believed that these mechanisms may have discouraged subsequent efforts to regenerate resid hydroprocessing catalysts through combustion techniques.
A number of processes have been suggested to immobilize or passivate the contaminant metals during oxidation to remove coke.
U.S. Pat. No. 4,089,806 discloses a regeneration process wherein a phosphorous component is incorporated onto the catalyst after the partial decoking and extraction step and prior to combusting the remainder of the coke from the catalyst. Incorporation of phosphorous components with the deactivated catalyst is thought to passivate or inhibit the formation of the undesirable vanadates.
U.S. Pat. No. 4,994,423 discloses a regeneration process wherein a Group IIA component is incorporated onto the catalyst after partially decoking the catalyst prior to combusting the remainder of the coke. The Group IIA metal is believed to passivate or inhibit the formation of the undesirable vanadates.
Partial decoking processes, however, are generally costly, complex, and difficult to implement and control. Moreover, it can be difficult to effectively disperse a passivating agent onto the surface of the catalyst when the catalyst is partially covered with coke.
Two-step oxidation has also been the subject of a recent U.S. Patent.
U.S. Pat. No. 4,975,399 discloses a process for regenerating a spent hydrotreating catalyst comprising one or more of molybdenum, nickel, tungsten, and cobalt on an alumina support comprising heating the catalyst at a first temperature ranging from 700.degree. F. to about 1000.degree. F. followed by heating the catalyst at a second temperature ranging from 1100.degree. F. to about 1700.degree. F.
It has been found that the two stage process described above can actually facilitate and increase catalyst attrition. Vanadium sulfide (V.sub.3 S.sub.4) can and generally decomposes to vanadium pentoxide (V.sub.2 O.sub.5) at oxidation temperatures above about 700.degree. F. Decomposition of vanadium sulfide to vanadium pentoxide subsequently catalyzes the detrimental attrition effects described above. Moreover, oxidation temperatures above 1400.degree. F. can cause the vanadium pentoxide to melt thereby increasing vanadium pentoxide migration and extending its exposure to and subsequent damage of the alumina support.
Group IIA metals have been used for retaining the crush strength of hydroprocessing catalysts.
U.S. Pat. No. 4,870,044 discloses treating a catalyst with a dissolved magnesium compound. The solute in the impregnating solutions can be any magnesium compound including, but not limited to magnesium nitrate, magnesium carbonate, magnesium sulfate, and the like. Although catalyst crush strength is an important property for use with fixed-bed reaction processes wherein catalyst particles located in the upper sections of the bed exert weight forces on catalyst particles located in the lower sections of the bed, it is less critical in ebullated bed processes where the random motion of catalyst particles caused by the upward flow of liquid and gas virtually eliminate such downward forces. In ebullated bed processes, the critical catalyst attribute is attrition resistance. It is well known that a catalyst that has a high crush strength does not necessarily have a high attrition resistance.
It has now been found that incorporating a Group IIA metal onto a large pore resid hydroprocessing catalyst prior to the deposition of the contaminant metals and coke and subsequent oxidative regeneration provides for superior catalyst attrition properties in the regenerated catalyst. This is believed to occur due to passivation of the contaminant metal by the Group IIA metal whereby conversion of the vanadium to vanadium pentoxide is preempted.
It has also been found that incorporating the Group IIA metal onto the catalyst prior to deposition of the contaminant metals and coke allows for better dispersion of the Group IIA metal onto the catalyst, thereby allowing for uniform deposition of the Group IIA metal onto the surface of the catalyst. The more uniform the deposition of the Group IIA metal, the greater the attrition resistance of the catalyst.
It has similarly been found that utilizing a two-step oxidation process whereby the first oxidation step is maintained at a temperature below 700.degree. F. provides a substantial reduction in conversion of vanadium sulfide to the highly oxidative vanadium pentoxide, thereby providing further improvements in catalyst attrition properties.
It has also been found that maintaining both oxidation steps at temperatures below 1400.degree. F. provides a further improvement in catalyst attrition properties by reducing vanadium pentoxide migration throughout the catalyst support.
It is therefore an object of the present invention to provide a catalyst regeneration method that does not substantially reduce catalyst attrition resistance.
It is another object of the present invention to provide a catalyst regeneration method that avoids costly, complicated, and time-consuming partial decoking and metals or catalyst component incorporation steps on-site.
Other objects appear herein.